With the aim of replacing metal parts for weight saving and cost reduction while having comparable or superior mechanical performance, structures based on composite materials comprising a polymer matrix containing a fibrous material have been developed. With this growing interest, fiber reinforced plastic composite structures have been designed because of their excellent physical properties resulting from the combination of the fibrous material and the polymer matrix and are used in various end-use applications. Manufacturing techniques have been developed for improving the impregnation of the fibrous material with a polymer matrix to optimize the properties of the composite structure.
In highly demanding applications, such as for example structural parts in automotive and aerospace applications, composite materials are desired due to a unique combination of light weight, high strength and temperature resistance.
High performance composite structures can be obtained using thermosetting resins or thermoplastic resins as the polymer matrix. Thermoplastic-based composite structures present several advantages over thermoset-based composite structures including the ability to be post-formed or reprocessed by the application of heat and pressure. Additionally, less time is needed to make the composite structures because no curing step is required and they have increased potential for recycling.
Among thermoplastic resins, polyamides are particularly well suited for manufacturing composite structures. Thermoplastic polyamide compositions are desirable for use in a wide range of applications including parts used in automobiles, electrical/electronic parts, household appliances and furniture because of their good mechanical properties, heat resistance, impact and chemical resistance and because they may be conveniently and flexibly molded into a variety of articles of varying degrees of complexity and intricacy.
With the aim of improving the manufacturing process for making composite structures and integrated composite structures and allowing an easier, shorter and uniform mixing or impregnation of fibrous materials, several ways have been developed to decrease the melt viscosity of the polymer matrix. By having a low melt viscosity, polymer compositions flow faster and are thus easier to process. By reducing the melt viscosity of the polymer matrix, the time needed to reach the desired degree of mixing may be shortened, thereby increasing the overall manufacturing speed and thus leading to increased productivity.
However, the use of a low melt viscosity polyamide composition for improving or accelerating the mixing or impregnation of fibrous materials may lead to composite structures that are not ideal for highly demanding applications such as the automotive field due to inferior mechanical and heat aging properties.
The addition of heat stabilizers to polymer matrix compositions can allow for a higher impregnation temperature which lowers the viscosity of the polymer matrix composition but these heat stabilizers can also interfere with adhesion of the overmolding resin.
U.S. Pat. No. 7,763,674 discloses a fiber reinforced polyamide composition heat stabilized with a copper iodide/potassium iodide mixture.
US 2010/0120959 discloses polyamide compositions comprising a transition metal ion-modified clay as a heat-stabilizer. The metal ion for use in modifying the clay is a transition metal selected from the transition metals in Group IB, VIIB, VIIB and VIII of the Periodic Table and combinations thereof.
US 2009/0269532 teaches a multilayer structure comprising at least one stabilized layer. The stabilized layer is stabilized with 0.5% stabilizer based on copper iodide and potassium iodide. This stabilizer is constituted of 10% copper iodide, 80% potassium iodide and 10% zinc stearate.
US 2008/0146718 discloses a non-fibrous-reinforced thermoplastic molding composition comprising a metal powder as a heat stabilizer wherein the metal powder has a weight average particle size of at most 1 mm and the metal in the metal powder is selected from the group consisting of elementary metals from Group VB, VIIB, VIIB and VIIIB of the Periodic Table, and mixtures thereof.
U.S. Pat. No. 7,811,671 discloses films which comprise polyamide compositions which use potassium iodide and cuprous iodide with a magnesium stearate binder as a heat stabilizer.
FR 2,158,422 discloses a composite structure made of a low molecular weight polyamide matrix and reinforcing fibers. Due to the low molecular weight of the polyamide, the polyamide has low viscosity. The low viscosity of the polyamide matrix allows an efficient impregnation of the reinforcing fibers.
U.S. Pat. No. 7,323,241 discloses a composite structure made of reinforcing fibers and a branched polyamide resin having a star structure. The disclosed polyamide having a star structure is said to exhibit a high fluidity in the molten state thus making possible a good impregnation of the reinforcing fibers so as to form a composite structure having good mechanical properties.
WO 2007/149300 discloses a semi-aromatic polyamide composite article comprising a component comprising a fiber-reinforced material comprising a polyamide matrix composition, an overmolded component comprising a polyamide composition, and an optional tie layer there between, wherein at least one of the polyamide compositions is a semi-aromatic polyamide composition.
However, there is still a need for an overmolded composite structure comprising a matrix resin composition that can rapidly and efficiently impregnate a fibrous material and wherein the overmolded composite structure exhibits good long-term heat stability and interlayer adhesion.